Field of the Invention
Embodiments of the present invention generally relate to a micro-electromechanical system (MEMS) digital variable capacitor (DVC).
Description of the Related Art
Some DVC devices are based on a movable MEMS element with a control-electrode above (i.e., a pull-up or pull-off or PU-electrode) and below (i.e., a pull-in or pull-down or PD-electrode) the movable MEMS element, as shown schematically in FIG. 1. In addition there is an RF-electrode below the movable MEMS element (i.e., plate or cantilever or movable plate electrode). During operation a voltage is applied to either the PU or PD-electrode, which causes the MEMS element to be pulled-up or pulled-down in contact to provide a stable minimum or maximum capacitance to the RF-electrode. In this way the capacitance from the movable element to the RF-electrode (which resides below the movable element) can be varied from a high capacitance Cmax when pulled to the bottom (See FIG. 2) to a low capacitance Cmin (See FIG. 3) when pulled to the top.
FIG. 4 shows how the MEMS DVC device is integrated in the Back-end-of-Line (i.e., BEOL) of a complementary metal oxide semiconductor (CMOS) process. A metal shield connected to RFGND (i.e., RF-ground) is placed underneath the MEMS DVC devices to shield the silicon substrate from the MEMS DVC device. This ensures that loss mechanisms in the silicon will not negatively impact the RF performance of the MEMS DVC devices. The metal shield is typically placed in the lower metal levels (e.g., M1). Additional metal levels M2 . . . Mn-1 between the metal-shield and the RF-electrode (implemented in Mn) ensure that the parasitic capacitance between the RF and the ground shield is limited.
FIG. 5 shows a top-view of the PD-electrode and RF-electrode of a MEMS DVC cell. In a typical MEMS DVC cell, the RF connection is made at side A of the MEMS DVC cell while the other connections (GND, PU, PD) are made at side B of the MEMS DVC cell.
FIG. 6 shows how multiple MEMS DVC cells are arranged around the RF-pin for optimal RF-performance. The CMOS waveform controller that controls the state of the MEMS DVC devices is placed in the same chip, either off to the side or underneath the MEMS cells.
FIG. 7 schematically shows the electrical connection of the waveform controller to the MEMS DVC cells. The movable element is typically on DC-ground and the voltages applied to the PD-electrode (Vbottom) and to the PU-electrode (Vtop) are typically controlled to ensure a long-life stable performance of the MEMS DVC device. The resistors Rpd and Rpu provide for isolation between the RF-signals present on the PU and PD electrode and the CMOS drivers. This also ensures no CMOS noise is coupled into the RF electrode of the MEMS DVC cells. In addition these resistors provide for damping of the MEMS devices within the MEMS DVC cells which allows for fast operation. Typically, these resistors are generated with high-resistivity poly-silicon and values of these resistors range from 50 kΩ to 10 MΩ.
FIG. 8 shows a cross-section of the MEMS DVC device near side B of the MEMS DVC cell. A connection is made between the poly-resistor and the PD-electrode to allow the CMOS waveform controller to apply voltages to each MEMS DVC cell while maintaining the isolation between the RF-signals and the CMOS signals. A hole is created in the ground-shield to allow the connection to the poly-resistor Rpd. A similar connection is made between the PU-electrode and the poly-resistor Rpu. Any noise present in the CMOS substrate can couple into the poly-resistor and subsequently couple into the PD and PU-electrode. This noise can subsequently couple into the RF-electrode and impact the RF-performance of the device.
To limit the noise in the substrate near the MEMS devices, substrate ground-contacts can be avoided near the MEMS DVC devices, so that any CMOS noise generated in the CMOS waveform controller that sits adjacent to the MEMS devices in the chip (See FIG. 6) has to travel some distance through the CMOS substrate before it reaches the poly-resistor. Substrate ground-contacts are not required in the region of the MEMS devices since there are no active devices in the silicon substrate in this region.
Because of the large-value poly-resistors, parasitic capacitances from this poly-resistor can affect the dynamic behavior of the signals applied to the MEMS cell. The poly-resistor will have parasitic capacitances to both the RFGND-shield above it and the substrate below it. FIG. 9 shows a simplified equivalent circuit model of the poly-resistors Rpu, Rpd of a given MEMS DVC cell with parasitic capacitors Cshield to the RFGND-shield and parasitic capacitors Csub to the substrate. The voltages Vtop, Vbottom are generated by the CMOS waveform controller with respect to the CMOS ground, which is also tied to the substrate. Any current coupled to the substrate through Csub has to travel through the substrate for a certain distance before the actual CMOS ground reference point is reached, i.e. there is a given series resistance Rsub. The current coupled to the RFGND shield is effectively directly coupled to the CMOS ground because the RFGND is tied to the CMOS GND either inside or outside the chip (indicated by the dotted line).
In a typical CMOS process the coupling of the poly-resistors to the substrate Csub can be larger than the coupling of the poly-resistor to the metal-shield Cshield above the poly-resistors. This means that the dynamic response of the poly-resistors will depend on the values of Csub and Rsub.
Each MEMS cell in FIG. 6 has a poly-resistor near side B of the cell to provide RF-isolation and MEMS-damping. Since each MEMS cell is located at a different position inside the chip, the value of Rsub can vary greatly from cell to cell. This means the various cells will exhibit a different RF-isolation and damping and also a different dynamic actuation of the various MEMS-cells on the chip.
There is a need in the art to eliminate this variation and to improve the RF isolation further.